As speed increases, somewhere around 45mph you can faintly begin to hear an oscillating humming sound. As speed continues to increase, it becomes more of a roaring sound. When I first heard it, it reminded me of a 4wd truck with off road mud tires. It was like I was on the rumble strips on the side of the highway with the tires.

From the driver's seat, it sounded like maybe a wheel bearing noise, but eventually I figured out that I could manipulate the throttle and make it change. It seems much louder coasting than accelerating, and it's pretty loud coasting as well. When going up a fairly step grade, it seems to quiet somewhat.

I just picked this rig up in Florida for a close friend and as I drove back to TN, I stopped and stayed the night with my sister and brother in law in the Birmingham Alabama area. I got my brother in law to ride along and he quickly walked to the back of the coach and said he could distinctly FEEL the vibration under the bathroom floor (above the rearend).

Being a long time drag racer who builds his own cars, my first thought was ring and pinion noise in the rear end, but driveshaft issues also crossed my mind. After talking with someone at the local Kenworth dealership after getting it home, he mentioned that air bag issues that cause the rear of the coach to sit too low, can cause excessive driveshaft angle and cause this kind of noise.

The coach is there (at Kenworth) now and they are draining the gear lube and having it analysed. They are also going to put their scope in and inspect the ring and pinion and bearings for any obvious signs of excessive wear or failure.

In the small tire drag cars we race, the rearend separates from the chassis under acceleration, and squats under deceleration. It would seem to make sense that the RV (too) when accelerating, the rearend separates (pushing the body upward), and when decelerating the body is at it's lowest point in relation to the rearend. If that's true, then I think the guy at Kenworth might have been correct. At least that's what I'm hoping.

I wondered if anyone here has ever dealt with a similar problem? and what did it turn out to be?

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Not that I'm any expert but when I first started reading your post I thought drive shaft. And from experience......when the rear of the DP is lower than spec ride height it will cause noises and vibration due to drive shaft angles.

If you can't find a driveline source for your sound you might consider this. I had a 02 Monaco that had a weird thrumming sound at certain speeds. We finally tracked it down to a windblast effect caused by "voids" around the front fiberglass cap from manufacturing. The dealership figured it out and sprayed a bunch of that expanding foam in the wheel wells and behind the headlights. This noise only occurred at a certain speed range and was independent of throttle useage. The airflow around the front of these moving houses can whistle and howl and vibrate making a lot of strange noises.

If you can't find a driveline source for your sound you might consider this. I had a 02 Monaco that had a weird thrumming sound at certain speeds. We finally tracked it down to a windblast effect caused by "voids" around the front fiberglass cap from manufacturing. The dealership figured it out and sprayed a bunch of that expanding foam in the wheel wells and behind the headlights. This noise only occurred at a certain speed range and was independent of throttle useage. The airflow around the front of these moving houses can whistle and howl and vibrate making a lot of strange noises.

Thanks for your response Dave. The frontal air wind noise problem was suggested by the dealer after I picked up the unit and began the drive from Florida back to TN last week. It seemed like a viable candidate until I realized I could modulate the noise with the throttle.

When I took Frank (my brother in law) for a ride, and he walked to the rear of the coach, he was able to quickly confirm there was distinct noise accompanied by vibration in the floor, around the area of the rear end. I'm really hoping this only excessive drive shaft angle caused by an air bag issue.

The more I've thought about it, the more I'm convinced it shouldn't be in the rear end itself (I.E. ring and pinion or bearing issue), due to the fact that the rear ends in these things come from commercial trucks that routinely carry more than twice the weight of even a fully loaded HR Imperial with a trailer behind it. The only way I can see rear end failure is if there was a pinion seal, or other leak and the rear end was run dry of lube for an extended period of time. Other than that, no matter how many miles this unit turns out to actually have on it, unless it's over 500K miles, I simply can't imagine the rear end failing.

Not that I'm any expert but when I first started reading your post I thought drive shaft. And from experience......when the rear of the DP is lower than spec ride height it will cause noises and vibration due to drive shaft angles.

What you describe sure sounds like an air bag issue. Haven't had that with the MH, but I'm a retired truck driver and experienced what you describe when the height lever assembly broke. The drive line misalignment caused by the air bags at the wrong height will make a noise that sounds and feels as if the rear end/transmission will come apart at any minute.
I'm with the KW mech and once he can check the height I believe you will be OK. Please keep us posted as to the solution.

I hope that its a driveshaft issue but;;;;
If you had a ride hight problem you would get louder noise under accell (load on shaft)
Driveshafts usually get noisy at 20 to 25 mph, (been there, done that)
The noise you descrip sounds like a shifting carrier in the rear end. This carrier is held in place by side bearings and when you deccell the ring and pinion contact changes causing vibration and noise.
you stated that the diff, rear axel is from a heavy duty truck, I think your MH has a 25k to 30k gvw and thats about it for the medium duty diff in your rig.
Again I hope I am wrong and its a driveshaft issue and await your shops diagnosis.

I will just throw this out to give you and other idea.
A few years ago I had my M.H. in the shop for one thing or another when I was informed that the rear seal on the end of the drive shaft was leaking so I had them replace the old seal with a new one.
Well, after I picked up my M.H. and drove off with it while I was going down the road I could hear a noise that I never heard before so I turn around and took it back to the shop.
They then sent a mechanic with me to see what it was that made the noise.
He told me that it was coming from the rear end.
To make a long store short the seal was not put all the way in after they reset it the noise disappeared.
Just a thought.

Scott,
I don't think its a 4 link issue !
My eagle had a similar issue and it was the Eaten Rear end howling between 60 and 65, in addition it had looser that normal U joints. I changed the joints and changed the rear gear fluid ( 80/90 wt ) at Speedco and the Howling all but subsided, it's still there but not near as loud and Eaten tells me that Howling is somewhat normal at different rear gear rpm's,all this was 90,000 mile ago.